An entire floor at the U.S. Court of Appeals for the D.C. Circuit was locked down Friday as judges heard arguments in a sealed case involving a grand jury subpoena that’s believed to be linked to the special counsel’s investigation.

The lockdown came as a three-judge panel considers a case arising out of a grand jury subpoena fight, suspected to be linked to Robert Mueller’s special counsel investigation into Russian interference in the 2016 presidential election. Details about the case—particularly the identity of the party hoping to suppress the subpoena—have been held under wraps.